Using CollectionManager you can edit or create your osu! collections in various ways:

• creating new collections using Beatmap listing view along with beatmap drag&drop support(drag beatmaps to a main window collection)
• Generate new collections from other players top100 rankings
• Merge(join) multiple collections into one

When creating this application my main intention was to be able to share ones collection with other people, and that's what it essentialy does: you can export your collections list to an .osdb file with other side can just import.
If they happen to be missing any beatmaps from your collection they'll be informed about it and be able to download missing beatmaps using built-in map downloader

it can also help you with generating beatmap lists in various formats like osu forums or reddit posts and others.